Add methods for detecting retriable and fatal Ably exceptions

common/src/main/java/com/ably/tracking/common/AblyHelpers.kt

@@ -221,3 +221,19 @@ private fun TokenAuthException.toAblyException(): AblyException =
         is CouldNotFetchTokenException ->
             AblyException.fromErrorInfo(ErrorInfo(message, 401, 100_002))
     }
+
+/**
+ * Indicates whether the exception from Ably is fatal and we should not attempt to retry it.
+ * Fatal errors have status codes like 4xx (e.g. 400).
+ */
+fun ConnectionException.isFatal(): Boolean {
+    return (400..499).contains(errorInformation.statusCode)
+}
+
+/**
+ * Indicates whether the exception from Ably is retriable and we can attempt to retry it.
+ * Non-fatal errors have status codes in range 500-504.
+ */
+fun ConnectionException.isRetriable(): Boolean {
+    return (500..504).contains(errorInformation.statusCode)
+}

common/src/test/java/com/ably/tracking/common/ConnectionExceptionTests.kt

@@ -0,0 +1,85 @@
+package com.ably.tracking.common
+
+import com.ably.tracking.ConnectionException
+import com.ably.tracking.ErrorInformation
+import org.junit.Assert
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.Parameterized
+
+@RunWith(Parameterized::class)
+class ConnectionExceptionTests(
+    private val statusCode: Int,
+    private val isFatal: Boolean,
+    private val isRetriable: Boolean,
+) {
+
+    companion object {
+        @JvmStatic
+        @Parameterized.Parameters(
+            name = "Status code: {0} | Is fatal: {1} | Is retriable: {2}"
+        )
+        fun data() = listOf(
+            params(
+                statusCode = 399,
+                isFatal = false,
+                isRetriable = false,
+            ),
+            params(
+                statusCode = 400,
+                isFatal = true,
+                isRetriable = false,
+            ),
+            params(
+                statusCode = 499,
+                isFatal = true,
+                isRetriable = false,
+            ),
+            params(
+                statusCode = 500,
+                isFatal = false,
+                isRetriable = true,
+            ),
+            params(
+                statusCode = 504,
+                isFatal = false,
+                isRetriable = true,
+            ),
+            params(
+                statusCode = 505,
+                isFatal = false,
+                isRetriable = false,
+            ),
+        )
+
+        private fun params(statusCode: Int, isFatal: Boolean, isRetriable: Boolean) =
+            arrayOf(statusCode, isFatal, isRetriable)
+    }
+
+    @Test
+    fun `exception should be treated as fatal if its status code is between 400 and 499`() {
+        // given
+        val exception = createConnectionException(statusCode)
+
+        // when
+        val result = exception.isFatal()
+
+        // then
+        Assert.assertEquals(isFatal, result)
+    }
+
+    @Test
+    fun `exception should be treated as retriable if its status code is between 500 and 504`() {
+        // given
+        val exception = createConnectionException(statusCode)
+
+        // when
+        val result = exception.isRetriable()
+
+        // then
+        Assert.assertEquals(isRetriable, result)
+    }
+
+    private fun createConnectionException(statusCode: Int): ConnectionException =
+        ConnectionException(ErrorInformation(0, statusCode, "Test exception", null, null))
+}